CRUSHING AND GRINDING 213 faces do not wear evenly, but soon corrugate, making it impossible to maintain a fixed discharge. Ball-tube mills may be used very efficiently for granulating. The reduction is accomplished by impact, if the device is properly operated, so that a minimum of undersize is produced. By using a short mill and a heavily forced feed, classifying the mill product and returning the oversize for further grinding, a uniform output is secured, together with minimum power expenditure. In such work it is necessary properly to proportion the size of the ball used to the size of the piece to be broken. Pulverizing.—In pulverizing, the work to be done is usually the reduction of particle size so that the product will pass through a wire screen having 100 apertures per linear inch or even smaller. Such service can be performed in ring-roll mills, machines of the Huntington type, Chilean mills, grinding pans, grinding disc, etc., but the tube mill, charged either with steel balls or flint pebbles, has by far the greatest use and is considered the most effective and efficient. In cement making, where a very fine product is obtained by dry grinding, tubes up to 8 ft. in diameter and 26 ft. long are used, making a product that is nearly all under 200 mesh and varying from that down to material that would pass a 1000-mesh screen, if such a screen could be made. Pulverized coal for fuel is so produced, and in mining the production of finely divided ores for cyaniding, flotation, etc. is so produced. A modification of the tube mill loaded with balls or pebbles is one that is loaded with steel rods (Marathon, Marcy, Cole and others). This arrangement acts as a multiple roll, grinding the material between the slowly turning surfaces. The machine is said to give excellent results, particularly in granulating. The charge of rods is carried above the horizontal axis of the mill, thus tending to balance it and reduce the amount of power consumed. Machines of this type have been in use experimentally for several years and recently have been installed on a large scale in one or two mining installations. Pulverization is the most expensive part of all rock reduction, more energy must be expended to accomplish it, and consequently this stage has received, and should receive, a maximum of study and attention. MISCELLANEOUS CRUSHING AND GRINDING MACHINES The stamp, either falling under the influence of gravity only, or driven by 'steam, has been of great importance in the development of gold and copper mining, but has no use, so far as known in other industries, except metallic junk treatment. It is an uneconomical method of crushing ore, and any one interested can find a detailed description and discussion in Richards "Ore Dressing.'7 The first crushing machine, after the war club, was the mortar and pestle. Except as a laboratory implement, this has become more of a mixing device and is described under "Mixing and Kneading/' The bucking board of the assayer may be considered a mortar that has been flattened out. The so-called drug mill consists of a cone revolving within another cone (or both surfaces may be curved) without eccentric motion, wherein it differs from the gyratory crusher. These surfaces usually have corrugations on them so that comminution is mainly a matter of shredding and tearing rather than of crushing or abrasion.